Lesson 6 - I2C IO Expander - ahmadsarraj.com

Motivation

In applications which consume many I/O’s (including LEDs, relays, sensors etc.), the MCU may not have enough GPIO’s to accommodate this. In such circumstances, an I2C expander can be used!

This project treats the MSP430 as an I2C expander which will supply 8 additional I/O’s to the host controller.

Background

The host controller writes to the MSP430 via the I2C bus. The I2C configuration for the host is:

Address slave with 7-bit address
Single master
0x48 slave address

The I2C communicates consists of a 3-byte message.

1st byte = Command

2nd byte = Data index

3rd byte = Data value

The MSP430 outputs the data to/reads the data from the I/O pins

Figure 1 - Wiring diagram — Figure 1 – Wiring diagram

The bit rate is approximately 100 kbps

The Code

The command byte tells the MSP430 what operation should be performed on the I/O’s. Table 1 below is list of possible actions:

Table 1 - All Command actions — Table 1 – All possible command actions

The data index byte determines which group or bit number to write to. Table 2 below lists out the possible actions:

The MSP430 splits the port into 3 groups:

Group 1 = Bits 0-3

Group 2 = Bits 4-5

Group 3 = Bits 6-7

Table 2 - All possible actions — Table 2 – All possible actions for Data Index

The data value byte contains the 8-bit value to be transmitted to the I/O port, group or bit. Table 3 below lists the possible actions:

Table 3 - All possible actions for Data Value — Table 3 – All possible actions for Data Value

Figure 2 - Code flowchart — Figure 2 – Code flowchart

The GUI

The GUI allows the user to monitor the I2C transactions.

E.g. if the 3-byte I2C transaction is 0x03, 0x02, 0x01 then the Set Bit command is executed. Bit 2 is selected and it toggles HIGH. In the GUI, the Set Bit row and Value updates to a 1.

Additionally, all of the commands can be toggled. Also, in the I/O Output Status block, each individual bit can be toggled